The present invention relates, in general, to a measurement system and to a method for monitoring a measurement system.
Particularly in the technical field of automation engineering, measurement systems, in particular incremental measurement systems, are used to determine a position of a moveable element, such as a motor shaft of a drive for driving a machine shaft. By means of an incremental measurement system in which the transmitter has a reference mark, it is possible after moving over the reference mark to determine the absolute position, in that the increments which have been moved over can be counted without any gaps by means of a counter after moving over the reference marks (zero position). The count of the increments which have been moved over in this case corresponds to a rough position, with a fine position then being determined by interpolation, for example as described in DE 27 29 697 A1. The rough position and the fine position together provide the absolute position.
Particularly in the case of machine tools, production machines and/or robots, measurement systems such as these are usually supplied during normal operation with electrical power from an open-loop and/or closed-loop control device which is used for open-loop and/or closed-loop control of moveable elements of the machine. If the open-loop and/or closed-loop control device is switched off, then, in many such measurement systems, the count for the increments which have been moved over is lost and/or increments which are moved over after switch-off are no longer also counted. Once the open-loop and/or closed-loop control device has been switched on again, the reference mark must first of all be moved over again in measurement systems such as these, in order to allow the absolute position to be determined.
EP 0 362 407 A1 attempts to address this problem by providing a battery which supplies the measurement system even after the open-loop and/or closed-loop control device has been switched off. Power is hereby not supplied to all the electrical components of the transmitter, but only a part of the transmitter is supplied with electrical power. The transmitter is thus operated in an energy-saving mode, rather than in the normal mode. It is possible, for example, to supply only a single RAM for storage of the count (however, in this case, any movements of the moveable element when the open-loop and/or closed-loop control device is switched off, can then not be detected). It is also possible, for example, for a light-emitting diode, which is otherwise operated continuously, of an optical transmitter to be operated only on a pulsed basis and for the interpolation electronics not to be operated at all, as a result of which the fine position is no longer detected at all or is no longer detected so well, as in the normal case, but always still to adequately count the increments moved over.
This prior art has the disadvantage that, if the power supply is inadequate in the event of failure of the battery, the count of the elements which have been moved over can be deleted or changed This can occur in particular in the event of an excessively long failure of the external power supply for the transmitter, which ensures the power supply for the transmitter during normal operation. Once the open-loop and/or closed-loop control device has been switched on again, and therefore after the external electrical supply voltage has been restored for normal operation of the transmitter, there is then no longer any guarantee that the position determined by the transmitter would actually match the actual position of the moveable element, because, in the meantime, the auxiliary supply voltage produced by the battery will have fallen too far to supply adequate power at least to the important electrical components of the transmitter.
DE 20 2004 000 413 U1 discloses a transmitter which includes a non-volatile memory to store the count, with an electromechanical snap-action mechanism being used as an independent energy source. This type of mechanism is very complex.
It would therefore be desirable and advantageous to obviate prior art problems and to allow identification of an incorrectly determined position caused by failure of the electrical power supply for the transmitter.